Canadian researchers report that life long bilingualism delays the onset of dementia. That's extraordinary, but not the first time that bilingualism has been linked to physical changes in the brain's structure. It was reported some time back that true bilinguals have notably denser brain matter in certain areas, a property shared with long-time London cabbies, of all things - this seems like a phenomenon ripe for investigation by the Freakonomics team.
Bilingualism is different from learning another language as an adult, regardless of level of fluency. Only languages learnt in the process of learning speech itself will contribute to the physical changes in the brain.